Ruby 2.4 documentation for this bracket syntax

Question

Have not used Ruby in quite sometime and ran into some unfamiliar syntax:

class AddQuantityToLineItems < ActiveRecord::Migration[5.1]

The [5.1] seems to be forcing a particular ActiveRecord::Migration version? What is this syntax called and where can the documentation for it be found?

Thanks!

Take a look at this question of SO: https://stackoverflow.com/questions/38543894/ruby-grammar-name-for-number-after-subclass-of-one-class — JesusTinoco, Aug 01 '17 at 22:05

score 4 · Accepted Answer · answered Aug 01 '17 at 22:09

It's a familiar syntax, but it's unfamiliar in this location, that's all. The base class specifier is allowed to be an expression, which can include method calls.

Here's a way of re-creating that situation:

class CrazyProxyClass
  def [](v)
     Class.new
  end
end

CrazyMethod = CrazyProxyClass.new

class CrazyDerived < CrazyMethod[1.2]
end

CrazyDerived.new
# => CrazyDerived

You can also get even more adventurous:

class NormalBase
end

class DebugBase < NormalBase
end

class Example < (ENV['DEBUG'] ? DebugBase : NormalBase)
end

The only limit is your imagination and tools like Rubocop that will tell you it's probably a bad idea to get this nuts. The only real obligation is that whatever that expression returns is a Class or you'll get a "superclass must be a Class" exception.

Ruby 2.4 documentation for this bracket syntax

1 Answers1